In this study the field effects on electrochemic ally controlled silicon etching in KOH during three- and four-electrode pn etch-stop have been investigated. When a p-doped bulk area is surrounded by n-doped silicon and the n-doped silicon is electrochemic ally passivated, small lines of bulk silicon in between the n-doped silicon would not etch. The size of the bulk silicon that did not etch could be moderated by the applied potential on the n-doped silicon during pn etching. For the three-electrode system the size of the electric field passivated bulk silicon could be controlled for line widths ranging between 60 and 100 mum. When a four-electrode system was used and the p-doped bulk silicon was forced to a more negative potential, it was possible to etch lines of bulk silicon down to 40 mum widths. This field-restricted pn etch-stop can be used for fabricating thin membrane structures with densely spaced n-doped regions that can be individually addressed by electronics. One application is the fabrication of neural sieve electrodes which comprise a pn etch-stopped membrane containing several phosphorous-doped individually addressable recording electrodes.